When did the events in לך לך happen?

If you just read parshas לך לך you would think that first Hashem told Avraham to go to Eretz Yisroel, then came the story with לוט, the 4 kings, etc. and then came the ברית בין הבתרים. However, the Tosafos (and other Rishonim) point out that if we take a closer look at the chronology from the medrashim we see that this is not true.

Hashem tells Avraham by the ברית בין הבתרים that his descendents will be in golus 400 years. Rashi points out that they were only in Egypt 210 years and therefore explains based on the medrashim that the 400 years started with the birth of Yitzchak. However, the Torah says in Parshas Bo that they were in Egypt 430 years. Rashi there explains (again based on medrashim) that the extra 30 years is from the ברית בין הבתרים, in other words the ברית בין הבתרים was 30 years before Yitzchak was born. We know that Yitzchak was born when Avraham was 100 years old which means that ברית בין הבתרים had to be 30 years earlier when Avraham was 70. However, at the beginning of parshas לך לך the Torah tells us that Avraham was 75 years old when he left Charan. This means that לך לך had to be 5 years after the ברית בין הבתרים. Tosafos in Shabbos 10b makes this calculation and says this is what happened. Avraham came to EY when he was 70 and went through the ברית בין הבתרים, and then he returned to Charan for 5 years until Hashem told him לך לך.

However, this is very strange for a number of reasons:

1. Why did Avraham come to Israel at the age of 70 and then leave?
2. לך לך was not the first time Avraham left his homeland, he already did it once before at the age of 70
3. Rashi and the medrashim make it out as if Avraham had no idea where he was going when Hashem told him לך לך, yet he had already been there.
4. Why would the Torah wrote the story in a backwards order?